Gyroscopic sensor

ABSTRACT

A gyroscopic sensor comprising: a sensing element ( 1 ) associated with detection and excitation electrodes ( 8 ); conductive rods ( 6 ) connected in particular to said electrodes ( 8 ); a protective housing ( 3, 4 ) enclosing the sensing element ( 1 ) and the electrode ( 8 ) and having insulating feed-throughs ( 7 ) for the conductive rods ( 6 ); and support means interposed between the housing ( 3, 4 ) and the sensing element ( 1 ) with the electrodes ( 8 ); the sensor being characterized in that said support means are constituted by the conductive rods ( 6 ) themselves, which are made so as to be elastically deformable.

The present invention relates to improvements applied to gyroscopicsensors comprising:

a sensing element associated with detection and excitation electrodes;

conductive rods connected in particular to said electrodes;

a protective housing enclosing the sensing element and the electrode andhaving insulating feed-throughs for the conductive rods; and

support means interposed between the housing and the sensing elementwith the electrodes.

In order to obtain good performance, gyroscopic sensors rely oncomponent parts of accurate shape, and on assembly that is extremelyaccurate; this gives rise to dimensional tolerances that are very tight,and to clearances that are very small.

Unfortunately, it is possible for at least some such component parts tobe made out of materials that are different, thereby leading tocoefficients of thermal expansion that can be very different. This givesrise to considerable difficulties in ensuring that the gyroscopic sensormaintains the required performance under varying ambient temperatureconditions, thus requiring assemblies that are suitable for allowing thecomponent parts to expand where necessary, while still complying withprecise values for clearances, spaces or air gaps, or complying withlimit values for stress that can be accepted by the component parts.

Furthermore, the sensitive element of a gyroscopic sensor is a memberthat is extremely fragile and very sensitive to mechanical shock, so itis desirable for it to be supported while being decoupled as much aspossible from mechanical shocks.

These difficulties arise, for example, in gyroscopic sensors where thesensitive element is a quartz resonator possessing one or more vibratingbranches and having detection and excitation electrodes in the form ofmetallization deposited directly on said branches. In that type ofembodiment, it is the piezoelectric nature of quartz that is used toimplement the excitation and detection functions.

Those difficulties arise most particularly in gyroscopic sensors havinga resonator in the form of a bell or a spherical cap, where such sensorsare presently undergoing considerable development. In that type ofresonator, the edge of the bell- or cap-shaped resonator is excited intoa mode of vibration that causes it to be deformed with components thatare both radial and tangential, and they also present a component ofdisplacement that is parallel to the axis of the resonator. Thus, suchgyroscopic sensors are known in which the radial vibration of the edgeof the resonator is detected (in which case, the bell or cap of theresonator is positioned to cover an electrode-carrier plate at least inpart, see, for example, U.S. Pat. No. 4,951,508), and gyroscopic sensorsin which axial vibration is detected at the edge of the resonator (inwhich case, the electrode-carrier plate faces the edge of the bell orcap of the resonator, see, for example, FR 99/05204).

Known resonators of that type, which originally had diameters of about60 millimeters (mm), have subsequently been developed so as to havediameters reduced to about 30 mm for high-performance spaceapplications.

More and more, it is being envisaged to use gyroscopic sensors withbell-shaped resonators in applications requiring lower performance andat manufacturing cost that is much smaller, for example controllingtactical missiles. Such applications are often characterized by the needto place a sensor unit (gyroscopes and accelerometers) in a volume thatis small, and in thermal and mechanical environments that are severe.Vibrating gyroscopes possess good qualities for such applicationsbecause of their small number of parts and their intrinsic robustness.

The key element for performance in a gyroscope having a bell-shapedresonator is the Q-factor of the resonator obtained by using silica tomake the vibrating bell. At present, silica is the only materialpossessing the qualities required for making a resonator havingQ-factors of an order of magnitude greater than several million.

Unfortunately, silica has a property which, while being favorable interms of stability in gyroscope performance, nevertheless gives rise toa difficulty in manufacture: its coefficient of thermal expansion isextremely small, being of the order of 0.5 parts per million per degreeCelsius (ppm/° C.). Gyroscopes are fixed on cores of metallic materials,often made of aluminum, having a coefficient of expansion of 23 ppm/° C.It is therefore necessary to use a special architecture in order toaccommodate the transition between silica and the metal material of thecore so that temperature variations do not disturb the operation of thegyroscope.

The resonator is used electrostatically with detection being capacitive,which, in order to be efficient, requires very small air gaps to beachieved (a few tens of micrometers (μm)). It is important to limitvariations in air gap size as caused by differential expansion betweenmaterials and by deformation of the parts. Conventionally, this leads tousing an assembly that possesses one degree of freedom (e.g. sliding ina plane as in the device of document U.S. Pat. No. 4,951,508), or tousing parts that are elastic.

For the newly-envisaged applications implementing a bell-shapedresonator, the environments are becoming more and more severe:temperature range of −40° C. to +90° C., and the ability to withstandshock or impact giving rise to accelerations of several hundred timesthe acceleration due to gravity (g). Furthermore, the available volumeis becoming smaller and smaller, which is leading to resonators in whichthe bell is of ever-decreasing diameter, which in present applicationsis about 20 mm, for example.

Under such conditions, conventional solutions are no longer suitable.

The object of the present invention is thus to propose a novelarchitecture for a gyroscopic sensor, in particular a sensor having abell-shaped resonator, which ensures dimensional stability of thesensing elements of the gyroscope in thermal and/or mechanicalenvironments that are severe and which, in particular, makes it possibleto use bell-shaped resonators of small diameter as desired in practice.

To this end, the present invention provides a gyroscopic sensor asspecified in the preamble which is characterized in that said supportmeans are constituted by the conductive rods themselves, which are madeso as to be elastically deformable.

For a gyroscopic sensor having a bell- or cap-shaped resonator, thesensor of the invention further comprises:

a resonator in the form of a circular symmetrical bell or cap andpossessing an axial fixing stem; and

an electrode carrier carrying said detection and excitation electrodesand cooperating with the resonator, the electrode carrier carrying theresonator via its fixing stem;

said protective housing containing the resonator and the electrodecarrier;

and said conductive rods forming the support means are interposedbetween the electrode carrier and the housing.

By means of such an arrangement, the mechanical assemblies havingload-bearing surfaces, which might rub against one another under theinfluence of external conditions (temperature, vibration, . . . )thereby dissipating energy which would degrade the Q-factor of theresonator, and thus the precision of the gyroscope, have purely andsimply been eliminated. The support function is now carried out bymembers (the conductive rods) that were already present and whosepresence is, in any event, necessary for providing electricalconnections to the resonator.

The dual function now carried out by the conductive rods makes itpossible to eliminate causes that disturb proper operation of thegyroscope, enabling space to be saved by omitting members that are nolonger needed, and thus making it possible to provide devices of smallerdiameters, while also enabling the unit cost of such devices to bereduced.

Advantageously, with bell-shaped resonators, the conductive rodsconnected to the electrodes are distributed symmetrically and circularlyaround the axis of the resonator stem.

Also advantageously and under the same circumstances, the sensor mayfurther comprise three conductive rods that are symmetricallydistributed around the axis of the resonator, one of these rods beingconnected to a guard ring provided on the electrode carrier and anotherof these rods being connected to metallization of the resonator; thethird conductive rod may serve merely to be present and contribute tosupporting the electrode-carrier plate, where its presence makes itpossible to avoid the electrode-carrier plate tilting relative to thegyroscope housing.

In a particular embodiment, the housing comprises a metal base and acover secured thereto, and the base is provided with said insulatingfeed-throughs for the conductive rods.

Finally, the dispositions adopted in the invention lead to the followingadvantages:

the sensing element of the gyroscope is on a suspended mounting with acutoff frequency that is easily adjustable, and it can move intranslation parallel to the sensing axis of the gyroscope without thisaxis becoming tilted, which would be harmful for gyroscope accuracy;

the mount is suitable for use both with radial detection resonators andwith axial detection resonators;

the mechanical and electrical connections are made by the same elements,thereby simplifying assembly and reducing cost;

deformations associated with shock or with localized temperature in theconductive rods do not affect the air gaps used in operating theresonator; and

since the base of the gyroscope is made out of the same material as thegyroscope support, there are no temperature constraints in how thegyroscope is fixed.

This architecture is very well adapted to making free gyros, inparticular those having small-sized bell-shaped resonators that aresuitable for use in mechanical and thermal environments that are severe.It can also be used for gyroscopes of larger size, and in environmentsthat are less severe, given that differential expansion is a problemthat becomes increasingly difficult with increasing dimensions.

The invention will be better understood on reading the followingdetailed description of embodiments given purely as illustrativeexamples. In the description, reference is made to the accompanyingdrawing, in which:

FIG. 1 is a diagrammatic side view in section of an embodiment of agyroscopic sensor having a bell-shaped resonator arranged in accordancewith the invention;

FIG. 2 is a diagrammatic section in side view of another embodiment of agyroscopic sensor having a bell-shaped resonator arranged in accordancewith the invention;

FIG. 3 is a plan view of the electrode-carrier plate of the FIG. 2resonator;

FIGS. 4 and 5 are fragmentary views showing the ability of theconductive rods of the gyroscopic sensors arranged in accordance withthe invention to accommodate deforming forces; and

FIG. 6 is a fragmentary view showing a variant embodiment of a portionof the gyroscopic sensors of FIGS. 1 and 2.

The description below relates more particularly to gyroscopic sensorshaving a bell-shaped or cap-shaped resonator, since the dispositions ofthe invention can be applied particularly advantageously in this type ofgyroscopic sensor, particularly those fitted with a resonator of smalldiameter, it being nevertheless understood that these dispositions alsoapply to the support for the sensing element of any type of gyroscopicsensor.

With reference initially to FIGS. 1 and 2, a gyroscopic sensor having abell- or cap-shaped resonator comprises four main elements:

a bell- or cap-shaped resonator 1, which can be hemispherical in shapein particular, as shown, and which possesses a fixing stem 5;

a part carrying the electrodes required for operating the resonator,referred to below as an electrode carrier 2, this part also having theresonator stem 1 anchored therein (the electrodes are not visible inFIGS. 1 and 2);

a base 3 enabling the gyroscope to be fixed on a support; and

a cover 4.

The resonator 1 and the electrode carrier 2 are made of silica so as toensure that the air gaps are stable, given that silica is a requiredmaterial for the resonator.

The electrode carrier 2 can have various configurations. It can behemispherical with electrodes facing the inside face of the resonator,as shown in FIG. 1. It can also be plane, with electrodes being placedfacing the end edge surface of the resonator, as shown in FIG. 2.

The resonator 1 and the electrode carrier 2 are assembled together toform the sensing element of the gyroscope. The air gaps between thesetwo parts are a few tens of μm. This subassembly is mechanically securedto the base 3 by support means. Thereafter, the cover 4 is installed soas to enable the resonator to operate in a secondary vacuum. Electricalconnections 6 are established between the electrodes and the controllingelectronics which is situated outside the gyroscope. These connectionspass via leaktight feed-throughs 7 that are also electrically insulatingand that are provided in the base 3.

In general, the base 3 is made of metal, having a coefficient ofexpansion which can either be close to that of silica, or else close tothat of the material from which the support is made and on which thegyroscope is fixed. In either case, differential expansion will occurbetween the base 3 and one of the parts with which it is assembled. Itis essential that the stresses induced by this differential expansion donot disturb the assembly comprising the resonator and the electrodecarrier, whether by varying the air gap as a function of temperature orby generating stresses that are too great in the silica. The tractionstrength of silica is very weak compared with that of a metal.

In accordance with the present invention, in order to assemble thevarious component parts together, the above-mentioned support means areconstituted by the above-mentioned electrical connections 6. In otherwords, once the above-mentioned component parts have been mechanicallyassembled together, it is the electrical connections 6 which serve tosupport the assembly comprising the resonator 1 and the electrodecarrier 2 on the assembly constituted by the base 3 having the cover 4secured thereto.

As shown in FIG. 3, the electrode-carrier part possesses variouselectrodes (e.g. eight electrodes in the example shown) constituted bythin metal deposits placed facing the resonator and enabling it to beoperated. For the FIG. 1 resonator, the electrodes make use of radialdeformation of the resonator. For the FIG. 2 resonator, they make use ofaxial deformation. These two configurations enable the same vibratorymodes to be used. The mass of the sensing element made in this way is afew grams, because silica is used.

Referring to FIGS. 1 and 2 in conjunction with FIG. 3, the gyroscopebase 3 is a circular part with eleven leaktight and insulatingfeed-throughs 7 having respective conductive rods 6 engaged thereinsuitable for providing the above-mentioned electrical connections:

eight rods 6 a (shown in FIGS. 1 and 2) serve to conduct the signalsfrom the electrodes 8 of the electrode carrier 2;

one rod 6 b (not shown in FIGS. 1 and 2) connects to the guard ring 9that separates the electrodes;

a rod 6 c provides a connection (not shown) with the metallization ofthe resonator; and

the last rod 6 d does not have any electrical function; it serves solelyto ensure that the electrode carrier remains in a stable orientation.

The eight rods 6 a connected to the electrodes 8 are placed on a circle10. The three additional rods 6 b, 6 c, 6 d are placed concentrically(circle 11) inside the first eight rods, at the vertices of anequilateral triangle.

Each leaktight and insulating feed-through 7 is made in conventionalmanner by sealing a metal rod 6 in the base 3 using glass. The base 3 ismade of the same material as the support on which the gyroscope is to befixed.

The ends of the eleven metal rods 6 are fixed to the electrode carrier 2by soldering (at 12) to the metal deposit made on the silica. This isequivalent to the electrode carrier 2 being mounted on “piles”, whichmeans that this part can move in translation parallel to the base 3.Such movement in translation is obtained by deforming the free portionsof the metal rods 6. In the event of acceleration or shock parallel tothe plane of the base 3, then deformation occurs as shown in FIG. 4. Inthe event of thermal expansion, then the deformation is as shown in FIG.5.

By modifying the length and the diameter of the free portions of themetal rods 6, it is possible to adjust accurately the flexibility of thesupport so as to localize deformation in the metal rods and limit theamount of stress that is transmitted to the electrode carrier 2, whetherunder the effect of mechanical shock or under the effect of thermalexpansion. Controlling the diameter and length of individual metal rodsis easier than, for example, controlling the thickness of a continuouspart having the same stiffness.

Thus, the resonator 1 and the electrode carrier 2 constitute a rigidassembly which is isolated from the outside by the suspension made usingthe metal rods 6. The resonant frequency of this suspension can belocated between the maximum frequency of external vibration and thenominal operating frequency of the resonator 1, thereby filteringdisturbances transmitted to the resonator. Similarly, in the event ofthe resonator being dynamically out of balance, the suspension willfilter the transmission of energy to the outside.

The figures shown are not limiting. The connections between the metalrods 6 and the electrode carrier 2 can be made in various ways, forexample by solder or by conductive adhesive. It is also possible to useintermediate inserts 12, e.g. made of Invar, which are fixed in thesilica part, as shown in FIG. 6. The number of metal rods can bemodified as a function of the number of electrodes or as a function ofthe internal connections made between the electrodes. It is necessaryonly to ensure symmetry that is as complete as possible so as to avoidthe sensing element tilting relative to the base, and so as to ensurethat movement takes place in the form of parallel translation, therebyconserving the direction of the inlet axis (measurement axis) of thegyroscope without giving rise to conical movements which cause agyroscope to drift.

In the context of the present invention, the metal rods 6 must be madeof a material or a set of materials that is not only a good conductor ofelectricity in order to perform the electrical conduction function, butthat also possesses a good coefficient of elasticity so as to be capableof performing the mechanical support function under the requiredconditions as set out above. By way of example, rods made of aniron-nickel alloy or of an iron-cobalt alloy (e.g. the alloys sold underthe reference VACON CF25 by Vacuumschelze GmbH) satisfy these tworequirements.

What is claimed is:
 1. A gyroscopic sensor comprising: a sensing elementassociated with detection and excitation electrodes (8); conductive rodsconnected in particular to said electrodes; a protective housingenclosing the sensing element and the electrode and having insulatingfeed-throughs for the conductive rods (6); and support means interposedbetween the housing and the sensing element with the electrodes;characterized in that said support means are constituted by theconductive rods themselves, which are made so as to be elasticallydeformable.
 2. A gyroscopic sensor according to claim 1, characterizedin that it comprises: a resonator in the form of a circular symmetricalbell or cap and possessing an axial fixing stem; and an electrodecarrier carrying said detection and excitation electrodes andcooperating with the resonator, the electrode carrier carrying theresonator via its fixing stem; said protective housing containing theresonator and the electrode carrier; and in that said conductive rodsforming the support means are interposed between the electrode carrierand the housing.
 3. A gyroscopic sensor according to claim 2,characterized in that the conductive rods connected to the electrodesare distributed symmetrically and circularly around the axis of theresonator stem.
 4. A gyroscopic sensor according to claim 2,characterized in that it further comprises three conductive rods thatare symmetrically distributed around the axis of the resonator, one ofthese rods being connected to a guard ring provided on the electrodecarrier and another of these rods being connected to metallization ofthe resonator.
 5. A gyroscopic sensor according to claim 1,characterized in that the house comprises a metal base and a coversecured thereto, and in that the base is provided with said insulatingfeed-throughs for the conductive rods.